Interesting article:
http://www.bostonherald.com/news/national/west/view/20091014scientists_have_learned_much_from_loma_prieta_quake/srvc=home&position=recent
Scientists have learned much from Loma Prieta quake
By Lisa M. Krieger / San Jose Mercury News | Wednesday, October 14, 2009 |
http://www.bostonherald.com | West
Photo
Photo by AP (File)
SAN JOSE, Calif. " The Loma Prieta quake was not merely about destruction " it was a force of creation, as well.
With one giant shrug, Scotts Valley ascended and Los Gatos sunk. Elsewhere around the Bay Area, the rupture triggered geologic shifts that altered the landscape in less perceptible, but just as profound ways.
Loma Prieta was a dream come true for scientists, ending decades of geologic tranquility. And because it occurred in a location so rich with instrumentation, they were afforded an unusual opportunity to collect seismic data and study its mechanics.
Using data gathered during the quake and in studies performed during the past 20 years, geologists have learned much about what happened, and why. Their work has improved understanding of the seismic threat in the bay region and contributed to more effective strategies to reduce losses, such as retrofitting bridges, rebuilding freeways, replacing gas and water pipelines, reforming building codes and improving coordination of emergency response systems.
"Loma Prieta showed us that what’s important is not just how an earthquake occurs, but our ability to predict what its effects will be," said Jack Boatwright of the U.S. Geological Survey in Menlo Park, Calif.
Although they will debate for years the detailed interpretation of recent studies, scientists from the USGS, Stanford, University of California and other institutions have made several key findings:
"The San Andreas Fault slipped in a fundamentally different way than it had in previous major earthquakes, such as the 1906 quake. Its motion included substantial lifting and sinking rather than the pure horizontal sliding motion normally seen on so-called "strike-slip" faults like the San Andreas.
Imagine the Pacific Plate leaning against the North American plate, like a ladder. In the quake, the Pacific Plate shoved over the North American Plate " causing as much as 24 inches of uplift along Soquel-San Jose Road, about one mile southwest of Summit Road, while down by the intersection with Blossom Hill Road, it dropped 4 inches.
"It’s like Scotts Valley trying to climb up a ramp over Los Gatos," said Paul Spudich of the U.S. Geological Survey in Menlo Park.
"Although stress was relieved by the quake, it was not a typical shallow San Andreas-type rupture, as initially thought, but a far deeper event " perhaps on a different strand of fault " that stirs only every several hundred years. It taught scientists that the San Andreas Fault Zone is riddled with a much more complex web of faults, some quite submerged.
But this means that the potential for a shallow event on the San Andreas Fault in the Santa Cruz Mountains may still exist because the earthquake may not have released all of the strain stored in rocks next the fault.
Inspired in part by Loma Prieta, a team led by Ned Field of the USGS published a major 2007 forecast that concludes there is a 99 percent chance that in the next 30 years Californians will experience one or more magnitude 6.7 or greater quakes and a 46 percent chance of a catastrophic 7.5 magnitude quake.
In the Santa Cruz Mountains, there is a 0.6 percent chance of that site repeating a Loma Prieta-size event in the next five years; within the next 30 years, the team predicted a 4 percent chance of such an event.
"Only about half of the earthquake’s vast energy was focused toward the urbanized San Francisco Bay region; the other half was focused toward the rural southeast. Had things been different, shaking in the bay region would have been both longer and stronger, causing far more damage.
Fear of greater loss has inspired the expansion of a USGS-based seismic recording system in structures, which will measure construction damage during the next quake and reveal ways it can be improved.
"It led to another startling finding, as well: Ground failure can be more damaging than shaking. Rocks may fall from cliffs, steep slopes slide, and even flat ground may crack and tilt. Landslides caused $30 million in earthquake losses, damaging at least 200 residences. The monthlong closure of Highway 17 because of a landslide further isolated the badly damaged community of Santa Cruz.
As a result, there is now improved surveillance of ground motion, using dectors " essentially, GPS receivers " to identify whether land is moving.
"It demanded greater respect of a phenomenon called "liquefaction," when sandy deposits become saturated with water " leading to sinking, tilting or the destruction of buildings, gas lines and water mains.
Liquefaction at 134 locations caused $99.2 million of the total earthquake loss of $7 billion. As part of a new program between the USGS and Pacific Gas &Electric, a sophisticated mechanized Earth probe is being used to determine whether the young sand deposits ringing the bay may liquefy during strong shaking. Their findings are being used to prepare a new detailed set of liquefaction hazard maps for the bay region.
Loma Prieta was the basis of the first set of recordings of damaging levels of shaking on a wide variety of geologic materials, including soft sand and clay. These records clearly document that ground shaking is much more violent on the soft sediments around the bay margins than on bedrock.
Strong shaking that was amplified by a factor of about two by soft soils caused damage as far as 60 miles from the epicenter. This improved understanding of the shaking hazard on soft ground has led to significant changes in building codes. It also spurred the state to pass the Seismic Hazard Mapping Act of 1990, which requires purchasers of property be told that if they buy undeveloped property, they will have to do special studies to investigate potential problems. Before Loma Prieta, state maps were regional and lacked specificity.
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Because of Loma Prieta, the USGS and California Division of Mines and Geology have produced a set of statewide shaking-hazard maps based on current knowledge of more than 200 active faults. The ShakeMaps depict geographic variations in the likely maximum severity of shaking to be experienced within a 50-year period. They’re serving as a principal foundation element for the seismic provisions of a new national building code.
And although scientists still can’t predict the timing of quakes, Loma Prieta inspired them to better project their effects. In 1995, the USGS and Association of Bay Area Governments collaborated to produce maps of ground shaking intensity for several scenarios. Maps of individual cities are now accessible on the Internet (www.abag.ca.gov/bayarea/eqmaps) to show the estimated shaking intensities superimposed on streets.
Finally, it forced the expansion of fast computer-generated information, which will help emergency response officials identify locations where damage and need are likely to be greatest. For years, computers at the USGS and the University of California, Berkeley, have posted quake information for Northern California. But the demand for information overwhelmed the capacity of the site. So it has been upgraded and its capacity expanded.
Now a location map is posted on the site within a minute of a quake; its magnitude is posted within five minutes (http://quake.usgs.gov/recenteqs). Maps showing the pattern of shaking can be posted within tens of minutes.
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Ongoing research will further our understanding of Bay Area quakes and the aftermath. A deep hole drilled directly on the San Andreas Fault near the Monterey County town of Parkfield is revealing the physical and chemical processes that control earthquake generation. Global Positioning Satellites are monitoring " minute by minute " the movement of plates. And a new 3-D computer model of the upper 20 miles of the Earth’s crust is helping researchers better predict the impact of shaking; because seismic waves propagate through different rock types with differing speeds, the waves are altered as they travel, causing a wide range of damage.
Loma Prieta was not "the big one" " it was a moderately big one, and far more complex than once believed, geologists say.
"The best advice," Field said, "is to presume something is going to happen."
